It has been proposed heretofore to make multi-ply paper by discharging onto a forming wire multiple jets of papermaking stock from the slice opening of a multilayer headbox comprising essentially a plurality of stacked headboxes in a common shell. Multilayer headboxes for forming a plurality of layers simultaneously were first proposed in German Pat. No. 899,896 to Waldhof and it was there recognized that the velocities of the multiple jets of stock must be coordinated to avoid any relative speed and thus any intermixing of the stocks as they leave the slice opening. The art ever since has regarded this condition as a requirement for layer purity.
The Beck U.S. Pat. No. 3,598,696 discloses a multilayer headbox comprising upper and lower walls and intermediate partitions forming separate channels for supplying different stocks or stocks at different velocities in sequential fashion through slice openings spaced apart in the machine direction to a solid or suction breast roll. Such machines in which the layers are laid down sequentially are limited to slow machine speeds and are not suitable for the production of light weight paper such as tissue.
In recent years, exceptional layer purity has been achieved by preventing the stock jets from coming together after leaving the slice opening and for a short distance in the direction of the forming zone therefrom by the use of so-called "air wedges" that are maintained between the jets as they leave the slice opening. Highly effective methods and apparatus utilizing such "air wedge" technology are disclosed in the copending applications of Eric Gunnar Stenberg Ser. No. 210,781, filed Nov. 26, 1980, and Ser. No. 266,174, filed May 22, 1981.